For costumes, well he may think of doing something I did in a prelude I was writing for my teen character Asher, and that is take what little money he had and raid a sporting good store for protective gear, and wear it over/under a hoody and baggy clothes :) But I think something else that may be more likely since this is a family tradition is for his costume to resemble the traditional garb of the warlocks of his bloodline. So, robe, plenty of talismans made from metal, stone and probably some plants. But adjusted for his theme of modernizing. Instead of a robe may be a long hooded coat that's easier to move in. Some armor dirt bikers use for racing, I've used it for making costumes irl and it's some good stuff. I would keep the traditional talisman setup, but he could add other things too, things that are techy but still old, like circa 1990's pagers and whatnot. :D

His spells depend on what you want to do in game I would think, your concept is a great one for marrying an rp concept with gameplay desires to get the most fun out of him. But witchcraft traditionally involved potions too, so maybe he has a part-time job at a coffee/tea shop where he can brew things in the latte machine? I don't think direct damage spells would be his mainstay, so as Atma said above, I'd think of him as a Guardian archetype first and foremost. I think Devices is the most thematic. If you wanted a more offensive role, control sets may be the way, unless you wanted out right dps.

Post launch I think a Manipulation skillset would be ideal.

That's why I thought about Manipulation, but that isn't a launch powerset, and I'm not even sure which archetype it falls under. The description of Devices seemed pretty broad though, but for abjuration either Barrier Generation or Preservation. Preservation would be my personal pick, just because it seems to have a wider range of effects, so you could protect, but also do some healing and have access to debuffs. Buffing/debuffing could be an application of Transmutation magic too.

Definitely tie it to their heritage, I think that could work out to be the most unique aspect of the character. Play it to the hilt. Also by doing that, I think the Arcane Sentinels could be the most likely group to be aligned with. If Brujo's magic is hereditary and rare, they'll want to preserve it and have it on their side. How Brujo feels about it is another matter, even if his family was aligned with them in the past, that may not be a tradition he holds up. Though I think they'd be more benign than a government agency, and the Regency feels like a Lovecraftian cult on the down low.

Well, I think if the relationship was a give and take one, the AS wouldn't give them an order but would request assistance. The sentinels could have lost personnel and/or artifacts as well. I think its a fine idea for a story hook, especially if the Regency really IS a Lovecraftian style cult on the d.l. Not only would you have the ritual in question, was it a success or failure? Consequences of either. You'd have the lost, and whatever may have happened to them.

Hard for me to say about the former. Could mix it, they all have some potential, with the possible exception of the spouse who married into the family, unless it was more of an arranged thing/lucky coincidence. Hector's is just the most potent of them. However him being the only person because the trait is so rare, it could put some distance between him and certain family members, or make some of them more protective after what happened to his cousin.

The graphic design, if magic is a strong family trait, they may see it as a waste of time, conversely, if it's not, then whoever is really protective of him and this rare gift, may encourage it more and want to downplay his active use of magic.

So you could end up with: For magic/against the art vs against magic/for art. The street art could be a different plot angle, some community people may see it as nothing more than graffiti and persecute him, get the cops involved etc. It's something that can play with culture and the mundane.

Enemies I see them dealing with. Creatures attracted to magic, people wanting to steal their secrets or their power, which could include groups like the 5 Dragons for the street level enemy. Otherworldly monsters who thing Hector and fam would taste good with ketchup and a side of tentacle.